1. Field of the Invention
The present invention relates generally to a telemetry apparatus and more particularly to electromagnetic (EM) isolation gap sub devices as used in well drilling and production (e.g. oil and gas) industry.
2. Description of the Related Art
EM telemetry is one method of communication used, for example, when exploring for oil or gas, in coal bed methane drilling and in other drilling applications. In a typical drilling environment EM carrier waves from an EM telemetry device are modulated in order to carry information from the device to the surface. Upon arrival at the surface, the waves are detected, decoded and displayed in order that drillers, geologists and others helping steer or control the well are provided with drilling and formation data.
EM telemetry is well understood as a downhole to surface means of communication. The carrier is normally established by producing an oscillating current across an electrically insulating gap in an otherwise continuous section of steel pipe located close to the drill bit. This current typically follows an electrical return path via the drilling fluid and the nearby associated earth formations. A small fraction of the formation current is detected at surface using an electrically short antenna as one node and the metal of the rig as the other, the signal between these two being amplified and filtered before being decoded and displayed as useful data.
A significant issue in the generation of downhole current is the structural integrity of the gap sub. It must be strong enough to withstand the rigours of the drilling environment local to the bottom hole assembly (BHA)—high torque, vibration, temperature and pressure—to name but a few. The gap sub must also be electrically discontinuous in order that a significant fraction of the generated current is preferentially forced to follow a path within the earth formations. Any reduction in this fraction will reduce the signal amplitude at surface. Thus the electrical discontinuity must be effective whilst retaining sufficient strength to cope with all of the severe mechanical stresses without undue wear or breakage.
Early gap sub designs and their precursors were simple and yielded poor performance by today's standards. Typical of a mechanical means of producing an insulated gap between two metal pipes is taught by McEvoy, U.S. Pat. No. 1,859,311 whereby two tapered male threaded pipes are joined by a short complementary female threaded tube. The problem addressed was the electrolytic corrosion of such pipes, and in particular corrosion of their threads when in the presence of oil and gas well drilling fluids containing contaminants such as acids, sulphur and salts. The solution was to isolate the threads of the pipes from each other by means of a thin coating of an electrically-insulating material applied to the threads. A similar problem associated with the corrosion of sucker rod threads was discussed by Goodner, U.S. Pat. No. 2,940,787, which discloses a similar electrically-insulating solution using materials such as epoxies, phenolics, rubbers, alkyds, all with high dielectric strength, but with the augmentation of an anti-rotation frictional retaining means between adjacent rods.
Another type of insulative gap between pipes and other such tubular members used for drilling or the production of oil or gas in drilled wells is exemplified by Krebs, U.S. Pat. No. 4,015,234, which shows a means by which a time-controlled switch contained within a drill pipe can cause current to flow in the nearby earth formations while drilling a well for producing a telemetry signal originating downhole and of such magnitude that it can be detected at surface. This patent teaches a means and method to implement a simple form of EM telemetry via the placement of pads or annular rings within the external wall of a drill rod, these being the electrical conductors that enable the discharge of a capacitor into the earth. The conductors are insulated from each other and the drill rod by an electrically-insulating material.
A further type of mechanical means for developing an EM telemetry signal downhole is typified by a much more complicated gap sub as taught by Logan et al., U.S. Pat. No. 6,050,353, which shows providing EM gap subs incorporating insulative and anti-rotation means that have a multiplicity of parts and subassemblies comprising metal, rubber, plastic and epoxy in an effort to exclude high pressure (up to about 20,000 psi) drilling fluid from the gap. This design tended to be expensive and difficult to build, and required frequent maintenance.
The improvement of dielectric insulating plastics that combine ease of use, high strength, high adhesion, corrosion resistance and excellent performance at high temperatures (150° C. and above) enabled a significant simplification in EM gap sub design. For example, Camwell et al., U.S. Pub. No. 2008/019190, teach that an extremely simple and practical gap sub comprising a single male tapered coarse thread cylinder coaxially threaded into a complementary single female tapered thread cylinder, said threaded sections being separated by an injection-moulded thermoplastic (such as polyetherimide, polyethylethylketone, polyetherketone or the like) will have adequate strength to resist the rigors of modern oil and gas drilling environments. The efficacy of such a design, based on McEvoy U.S. Pat. No. 1,859,311 and Goodner U.S. Pat. No. 2,940,787, relies on the strength of modern stainless steels and modern thermoplastics as well as its simplicity—the gap sub being basically a three-component device, comprising two conductive cylinders separated by a coaxial dielectric cylinder. The devices use simple anti-rotation means being implemented by machining grooves and the like into the threaded sections, and relying on the high mechanical stress performance of the thermoplastic being able to resist relative torque between the threaded sections, once the sub is thermally cured after injection.
It is in the assembly of such a sub that difficulties arise. FIGS. 1 and 2 of US patent application 2008/0191900 A1 show the two overlapping threaded sections electrically separated by the dielectric material. To inject the dielectric the two conductive cylinders must be held within an injection moulding machine. Furthermore, the two conductive cylinders must be mutually threaded but must not touch in order that the injected plastic is able to form an effective insulative barrier with respect to the two cylinders. To this end the cylinders must be held mutually parallel, coaxial, threadably overlapping but ideally with the threads axially and radially spaced equally apart. These constraints form a significant mechanical fixturing complexity and require a tedious alignment and fixturing procedure. Yet further, the injection process is typically performed at 20,000 psi, and such pressures produce large axial and radial forces on the cylinders. Substantial means must therefore be employed to clamp both cylinders accurately and immovably within the mould such that lack of perfect simultaneous and symmetrical plastic injection through the various sprue passages in the mould do not move one conductive cylinder with respect to the other and cause an electric connection, thereby defeating the purpose of the gap in the sub.